The present invention relates to an apparatus and method for converting signals. More particularly, the invention relates to the technique of converting a signal encoded in the first format, into a signal encoded in the second format.
FIG. 1 shows a conventional apparatus for converting an image-coding format. The apparatus is designed to convert an input bit stream of image codes into a bit stream of a different image-coding format. More specifically, the image-coding format converting apparatus of in FIG. 1 converts an input bit stream of MPEG2 (Moving Picture image coding Experts Group: ISO/IEC13818-2) image codes, into a bit stream of MPEG4 (ISO/IEC14496-2) image codes.
In the image-coding format converting apparatus shown in FIG. 1, an input bit stream of MPEG2 image codes is supplied to the MPEG2 image decoder 210.
The MPEG2 image decoder 210 decodes the input bit stream of MPEG2 image codes, in accordance with the MPEG2 image-decoding scheme, thus generating an image signal. The image signal is input to the resolution/frame rate converter 211.
The resolution/frame rate converter 211 converts the image signal to an image signal that has different resolution and frame rate. The signal the converter 211 has generated is supplied to an MPEG4 image encoder 212. It should be noted that the converter 211 reduces the resolution of the decoded image signal to half the original value in both the vertical direction and the horizontal direction.
The MPEG4 image encoder 212 encodes the image signal supplied from the resolution/frame rate converter 211, in accordance with the MPEG4 image-encoding scheme, thereby generating an MPEG4-encoded bit stream. The MPEG4-encoded bit stream is output from the image-coding format converting apparatus of FIG. 1.
The MPEG4 image encoder 212 detects a motion vector from the image signal generated by the MPEG2 image decoder 210 and predict a motion from the motion vector, in the same way as in the ordinary encoding process.
The amount of data processed to detect the motion vector in the course of encoding the input image signal occupies about 60 to 70% of all data processed in the conventional method of converting an image-coding format. Consequently, it is difficult to process the image signal in real time. This will inevitably result in a time delay. Further, the image-coding format converting apparatus needs to be large and complex.
In the image-coding format converting apparatus described above, the resolution/frame rate converter 211 changes the resolution and frame rate of the image signal that the MPEG2 image decoder 210 has generated by decoding the bit stream of MPEG2 image codes in accordance with the MPEG2 image-decoding scheme. Then, the MPEG4 image encoder 212 encodes the image signal thus processed by the converter 211, in accordance with the MPEG4 image-encoding scheme, and generates an MPEG4-encoded bit stream.
Therefore, the resolution and frame rate of the image signal output from the resolution/frame rate converter 211 may not be appropriate, making it difficult for the MPEG4 image encoder 212 to perform MPEG4 image encoding correctly.